1. Field of the Invention
This invention relates to a stator assembly of a rotary electric device and a method of assembling the same and, more particularly, it relates to a stator assembly of a stepping motor and a method of assembling the same.
2. Description of the Prior Art
A stepping motor has a wide variety of applications including OA device and computer peripheral devices and, in most cases, it is an indispensable component of such devices.
While stepping motors are classified into PM, VR and HB types depending on the structure of the rotor, they commonly comprise a rotor that revolves inside and which is borne by a stator assembly having a number of stationary magnetic poles.
FIG. 8 of the accompanying drawings is an exploded perspective view of a known stepping motor. Referring to FIG. 8, it comprises a bottomed cylindrical stator assembly 51 having an inner structure as will be described in detail hereinafter. The stator assembly 51 carries a terminal section 53 on the outer periphery. A number of wires (not shown) are soldered to the terminal section 53 to connect the motor to external circuits. The stator assembly has yokes 55 made from magnetic metal plates as outer peripheral members thereof, upper and lower exposed magnetic poles 57 and 59 arranged respectively on the upper and lower halves of the inner periphery thereof and a number of alignment projections 61 arranged on the top of the assembly for centering.
The stepping motor also comprises a rotor 63 designed to revolve inside the stator assembly 51 and carrying on the outer periphery thereof N and S magnetic poles and a top plate rigidly secured to the top of the stator assembly 51. A bearing 69 for carrying the rotary shaft 67 of the rotor 63 is rigidly fitted to the center of the top plate 65. Alignment through bores 71 are arranged on an imaginary circle around the bearing 69 at positions corresponding to respective alignment projections. Each of the alignment through bores 71 is provided with a projection 73 for welding a resistor there standing downward toward the stator assembly 51 at a position away from the bearing 69 along the periphery thereof.
FIG. 9 is a sectional side view of the stepping motor of FIG. 8 having the above described components The stator assembly of the stepping motor additionally comprises an upper stator coil 75 and a lower stator coil 77. For assembling the stator assembly 51, a subassembly consisting of a pair of cup-shaped yokes 55 carrying a number of stationary magnetic poles and an upper stator coil 75 arranged inside the yokes and another subassembly consisting of a pair of cup-shaped yokes 55 carrying a number of stationary magnetic poles and a lower stator coil 77 arranged inside the yokes are laid one on the other in an injection molding type metal mold and then synthetic resin is injected into the inside so that the stator assembly 51 is assembled and produced as a unitary entity. A number of alignment projections 61 are formed with synthetic resin at the same time. Note that reference numeral 79 in FIG. 9 denotes synthetic residual resin remaining in the injection port of the metal mold and reference numeral 81 denotes a bearing held in position by a block of synthetic resin 83 injected into the stator assembly. As seen from FIGS. 8 and 9, the stator assembly 51 is realized by bonding an upper stator subassembly A and a lower stator subassembly B, which will be described in detail hereinafter.
FIG. 10 is an exploded perspective view of the above described stator assembly 51. The illustrated components are identically formed and laid one of the other to produce vertically arranged mirror images before they are bonded together to make a unitary entity. As seen from FIG. 10, the lower stator subassembly B comprises a lower cup 60 provided with a plurality of lower stationary magnetic poles 59a standing upward from the bottom thereof and a lower stator coil 77 arranged in the inside thereof. A yoke plate 55a provided with a plurality of suspended lower stationary magnetic poles 59b and projections 59c is secured to the top of the lower cup 60 from above with the projections 59c fitted into corresponding recesses. Under this condition, a terminal section 53 of the lower coil 77 comes engaged with a deepest central section 60c of a notch 60a of the lower cup 60. Note that the yoke plate 55a is provided with an alignment bore 55b and an alignment projection 55c. The upper stator subassembly A is assembly in much the same way as the lower stator subassembly B. FIG. 11 is a plan view of the lower stator subassembly B.
For aligning a yoke plate 55a with a lower cup 60 of a rotary electric device having a configuration as described above and fitting the former into the latter, the alignment projection 59c of the yoke plate 55a is fitted into the notch 60a of the lower cup 60 and projections 59d of the yoke plate 55a are engaged with the deepest central section 60c of the lower cup 60. Then, for placing the upper stator subassembly A on the lower stator subassembly B in position, the alignment projection 55c arranged on the yoke plate of the upper stator subassembly A if fitted into the alignment bore 55b formed in the yoke plate of the lower stator subassembly B and the alignment bore 55b formed in the yoke plate of the upper stator subassembly A is engaged with the alignment projection 55c on the yoke plate of the lower stator subassembly B.
With the above arrangement, the alignment projection 59c of the yoke plate 55a of the lower cup 60 and the matching notch 60a of the lower cup 60 have respective tolerances, while the alignment projection 59d of the yoke plate 55a and the matching deepest section 60c of the lower cup 60 have respective tolerances. On the other hand, the alignment projection 55c of the yoke plate of the upper stator subassembly A and the matching bore 55b of the yoke plate of the lower stator subassembly B have respective tolerances, while the alignment projection 55c of the yoke plate of the lower stator subassembly B and the matching bore 55b of yoke plate of the upper stator subassembly A have respective tolerances.
Thus, when the upper and lower stator subassemblies are bonded together with synthetic resin in a mold, the above tolerances can add up to a significantly large error for the positions of the stationary magnetic poles and a reduced alignment accuracy of the components that can by turn result in a poor performance of the rotary electric device.
In view of the above identified problems and other problems of conventional electric motors of the type under consideration, it is therefore the object of the present invention to provide a stator assembly comprising stationary magnetic poles arranged with an enhanced positional accuracy as compared with those of conventional electric motors and a method of assembling the same in a simple manner.
According to the invention, the above object is achieved by providing a stator assembly of the rotary electric device comprising a yoke provided with a plurality of stationary magnetic poles and a yoke plate bonded to said yoke by means of synthetic resin in a mold to form an integral unit for enclosing a stator coil, characterized in that it has small holes formed by alignment pins in the molded synthetic resin.
According to the present invention, there is also provided a method of assembling a stator assembly of a rotary electric device comprising a yoke provided with a plurality of stationary magnetic poles and a yoke plate bonded to said yoke by means of synthetic resin in a mold to form an integral unit for enclosing a stator coil, characterized in that it comprises a step of bringing an alignment pin arranged on a jig for combining the yoke and the yoke plate into engagement with a corresponding alignment sections of the yoke and the yoke plate to put the yoke and the yoke plate together in order to produce a unitary stator assembly and a step of placing the unitary stator assembly of the yoke and the yoke plate in an injection metal mold with said alignment section fitted to an alignment pin arranged in the mold for mutual engagement and injecting synthetic resin into the mold to bond the yoke and the yoke plate together.
With the above arrangement, an alignment pin arranged on a jig for combining the yoke and the yoke plate is brought into engagement with a corresponding alignment sections of the yoke and the yoke plate to put the yoke and the yoke plate together in order to produce a unitary stator assembly and thereafter the unitary stator assembly of the yoke and the yoke plate is placed in an injection metal mold with said alignment section fitted to an alignment pin arranged in the mold for mutual engagement and synthetic resin is injected into the mold to bond the yoke and the yoke plate together.